Fillers are compounding agents to be mixed in a rubber for the purpose of reinforcement or bulking of the rubber, imparting a special function to the rubber, etc. Carbon black, a representative filler, not only contributes to enhancement of the physical properties (reinforcing effect), such as elastic modulus and breaking strength, of a rubber, but also has a function of imparting conductivity, etc.
To obtain a reinforcing effect for rubbers similar to carbon black and obtain a rubber composition having low exothermicity, i.e., low loss characteristics, a method of using an inorganic filler such as silica is known, and has been applied, for example, to rubber compositions for environmentally friendly, fuel-efficient tires, etc.
In an inorganic filler-blended rubber composition, an inorganic filler, especially hydrophilic silica having a silanol group on the surface, blended therein agglomerates in the rubber composition due to its low affinity for a rubber, which is hydrophobic. Thus, it is required to enhance the affinity of silica for a rubber to enhance the reinforcing capability of silica and obtain an effect of low exothermicity. Known examples of such methods include use of a synthetic rubber the affinity of which for inorganic fillers is enhanced through end group modification with a polar group (see Patent Literature 1) and use of a synthetic rubber the affinity of which for inorganic fillers is enhanced through copolymerization of a polar group-containing monomer (see Patent Literature 2). Known examples of methods for modifying a natural rubber to introduce a polar group include a method in which a natural rubber is oxidized and then modified with a hydrazide compound having a polar group (see Patent Literature 3) and a method in which a silane coupling agent is added to a rubber composition containing a modified natural rubber having a polar group introduced and silica to further enhance the dispersibility of the silica (see Patent Literature 4).